Plywood structures are well known in the art. They are composed of layers or plies of wood veneer with adjacent plies bonded with an adhesive. Decorative plywood laminates include a decorative ply adhered to a plywood substrate. The ply of the substrate opposite or distal the decorative ply is known as the back ply. One of the limitations of the prior art plywood is its relatively poor dimensional stability, which results in warping and cracking.
Those skilled in the art have tried to improve the dimensional stability of plywood by various means. Most commonly the direction of the grain of adjacent plies have been aligned substantially perpendicular to each other. The plies having grain perpendicular to the grain of the decorative ply being crossband plies. Typically, the grain of the decorative veneer or ply is parallel to the lengthwise edge of the veneer.
Improved dimensional stability can be obtained by increasing the number of plies or decreasing the thickness of the plies. For a given desired thickness, the thinner the plies, the more plies comprise the plywood and the stronger and more stable the plywood.
Most plywood is made using the same species of wood. However, Hsu U.S. Pat. No. 5,040,582 discloses a multi species laminated veneer lumber (a specialty plywood used for lumber manufacture), as well as laminated veneer lumber made from the combination of hardwoods and softwoods. Hsu notes the difference in shrinkage and expansion between the different wood species, particularly between high and low density wood species. He teaches that the species sequence from face to core must be identical or very similar on either side of the center ply when an odd number of plies are used or on either side of the center line when an even number of plies are used.
Walser et al. U.S. Pat. No. 5,234,747 teaches a symmetrical lay-up of, from top to bottom, multiple plies of a low density, 6% to 15% moisture content spruce, multiple plies of a high density, less than 5% moisture content Douglas fir, and multiple plies of a low density, 6% to 15% moisture content spruce.
Polymer laminates comprising phenolic resin-impregnated paper, such as taught in Nelson et al. U.S. Pat. No. 5,989,668 have a dimensional stability superior to that of plywood. The dimensional stability of the resin-impregnated laminates permit the use of click-lock edge structures such as disclosed in Pervan U.S. Pat. No. 6,023,907 and Morian et al. U.S. Pat. No. 6,006,486.
As such, a need exists for a plywood laminate having improved stability and resistance to warping and delamination, particularly one that uses less expensive wood plies. Such a plywood laminate would permit the use of click-lock edge structures.